1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device generally means all types of devices that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
A metal oxide silicon field-effect transistor (MOSFET), which is formed over a silicon substrate, has been applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device).
A MOSFET including a silicon semiconductor has been miniaturized in order to improve its element characteristics. A short channel length of a MOSFET improves cutoff frequency f, which increases its operation speed. Moreover, the degree of integration of the MOSFET is improved.
However, there is a problem in that as the channel length of the MOSFET is shorter, a short-channel effect, such as a punch-through phenomenon or a change in threshold voltage, occurs.
Various countermeasures against the short-channel effect have been proposed. For example, when the thickness of a gate insulating film is small, a gate electrode layer can be close to a channel region, and thus influence of a gate electrode on the channel region is enhanced, which can suppress the short-channel effect. Accordingly, reduction in the thickness of the gate insulating film, which suppresses the short-channel effect, has been used as an effective technique for the MOSFET.
However, reduction in the thickness of the gate insulating film (3 nm or less, for example) causes a problem of a tunnel current passing through the gate insulating film. To solve this problem, study in which instead of silicon oxide, a high-k material (e.g., a hafnium oxide) which has a higher permittivity than silicon oxide is used as a material of the gate insulating film has been conducted (e.g., see Patent Documents 1 and 2). With the use of the high-k material, effective thickness which is obtained by conversion into a film thickness of silicon oxide (referred to as equivalent oxide thickness (EOT)) of the gate insulating film can be reduced (to 3 nm or less, for example) while physical thickness thereof can be large enough to prevent occurrence of a tunnel current.
Furthermore, in general, a countermeasure called channel doping in which a small amount of an impurity element such as phosphorus or boron is added to the whole of a shallow channel formation region of the MOSFET has been employed (e.g., see Patent Document 3).